The long-awaited opening of the National September 11th Museum, in time for the 11th anniversary of the terrorist attacks, could be delayed due to a simmering financial dispute between the Port Authority and museum officials.

Sources tell NY1 museum construction at the World Trade Center Site has slowed because of hundreds of millions of dollars in cost overruns, including security.

Sources also say the Port Authority stopped approving new contracts in September and is now claiming the foundation owes the agency $156 million in extra charges.

Museum officials, according to sources, deny they are responsible for those overruns and say they are owed $100 million by the Port Authority.

In a statement, museum officials said they are working with the Port Authority to come up with an immediate resolution that would allow the project to move forward.

Port Authority officials said they are cooperating to resolve the issue.